Adsorption of Paraffin Vapour on Oxidized Molybdenum Substrates at Nano and Micro Scales

Sputtered oxidized molybdenum surfaces were exposed at room temperature for different times to paraffin vapors obtained at 150◦C. Scanning polarization force microscopy (SPFM), optical and confocal microscopy were used to characterize the surfaces. The condensed morphologies are complex and strongly dependent upon the quantity of vapor molecules deposited on the substrate surface. A thin paraffin film is initially formed and quite uniform nano-height drops are nucleated randomly over it within 10–20 s time exposures. Their average contact angle ranged between 1◦ + –2.5◦. Further vapor deposition led to a more complex regime where nano-height drops do not show a clear interface with the film, while micro-sized drops do. The tangent approximation method adopted by Salmeron and Xu for the nano-drop regimes was extended to the micro-sized drop regime obtaining an averaged effective contact angle equal to 4◦ + –5◦. Both nano-height and micro-sized drops shape and effective contact angles have been discussed taking into account their interactions between the film and the drops.